Displays using liquid crystals have been proposed for generating color displays (see for example, U.S. Pat. Nos. 5,359,345 and 5,724,062 to Hunter). However, these patents require arranging individual pixels in rows and corresponding columns, (column 4, lines 36-39). The devices described can be expensive and complicated to manufacture, and can have narrow angular view ranges with low brightness. Additional display systems have been proposed with similar problems to those described above (see for example, U.S. Pat. No. 4,791,415 to Takahashi; U.S. Pat. No. 4,871,231 to Garcia, Jr.; U.S. Pat. No. 5,184,114 to Brown; U.S. Pat. No. 5,192,946 to Thompson et al.; and U.S. Pat. No. 5,317,348 to Knize).
Several patents have been proposed for displays using two-frequency up-conversion fluorescence (see for example, U.S. Pat. Nos. 5,684,621; 5,764,403; 5,914,807; 5,943,160; and 5,956,172 all to Downing). The Downing '403 patent appears to be the most relevant to the subject invention. Downing '403 is primarily concerned with embodiments where the use of different layers for red, green and blue emitters, abstract, FIG. 6, and briefly describes some mixing of only crystal type materials in a single display media. However, for the single display media, Downing '403 uses nanometer sized particles, column 4, lines 33+, column 9, lines 42-45, which would inherently be difficult to form, handle and disperse in a display medium.
Other relevant known patents such as U.S. Pat. No. 5,003,179 to Pollack; 5,051,278 to Paz-Pujalt; U.S. Pat. No. 5,154,962 to Mertens et al.; U.S. Pat. No. 5,245,623 to McFarlane; U.S. Pat. No. 5,622,807 to Cutler; U.S. Pat. No. 5,846,684 to Paz-Pujalt et al. also fail to overcome the problems with the other patents described above.
The concept of frequency up-conversion (UC) of infrared-to-visible light in rare-earth (RE) doped materials was reported more than forty years ago for the first time. The efficiency that was observed or expected for this process was low in singly doped media, but it was quickly noticed that the mechanism could be made one or two orders of magnitude more efficient by using ytterbium (Yb) as a sensitizer ion in addition to the active ion: erbium (Er), holmium (Ho), or thulium (Tm). Efficient UC materials were extensively investigated, as they could be used for several potentially useful photonic applications, such as in UC lasers (visible lasers that are pumped by infrared diode lasers), or in display applications. However, because no powerful source existed in the 980-nm region in order to excite those up-converters, no practical product came out of the research. With the development of powerful 980-nm diode lasers lead by the telecommunication industry, a technology that appeared to be too inefficient in the past now has legitimate practical applications.
It has been noticed in the past that pumping conditions caused heating of the material and that higher efficiencies were obtained with low duty cycle excitation. It was also reported that for a same average input power, higher efficiencies were expected in pulsed excitation mode than in continuous wave (cw) excitation due to the quadratic nature of the process. The effect of the pumping conditions for display applications of UC materials need to be understood, as several technologies might be used to form the image. The infrared source can either be scanned (vector-addressed or raster-scan), or the image can be directly projected using Digital Micromirror Devices (MEMS) such as in the Texas Instrument Digital Light Processing (DLP™) technology. In the latter case the materials would be undergoing pulse-excitation, whereas they would be quasi-continuously excited in the second case.
As earlier noted, the development of powerful diode lasers emitting near 980-nm by the telecommunication industry is an enabling technology that allows up conversion to be used in displays. In the past no good pump source was available. Now these diodes provide for practical applications when the efficiency of up-conversion materials are enhanced to provide useful levels of fluorescence.